Characteristics of included studies

This systematic review included observation [17] and cross-sectional [20] studies, a single-blind randomized controlled trial [16], a research article [19], and two studies [15, 18] that did not specify their type of study. 

Risk of bias within the studies

All 6 included studies were evaluated qualitatively by the tools of Cochrane Collaboration for the risk of bias (Figure 2). The assessment of the risk of bias was carried out independently by two reviewers. Two studies [17, 19] had a high risk of bias in incomplete outcome data. A high risk of bias in selective reporting was observed only in a study by Wiest et al. [17]. The highest proportion of low risk of bias included other biases, allocation concealment, incomplete outcome data, and selective reporting. Meanwhile, blinding of participants and personnel, and blinding of outcome assessment were noted as the highest proportion of unclear risk bias.

Clinical characteristics of TMD patients

A total of 417 patients were potential participants in TMD-related clinical trials, of which 302 patients were included in intervention groups. TMJ dysfunctions were diagnosed according to the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) method. RDC/TMD is a widely used diagnostic method for TMD assessment and it is conducted of two components: Axis I, which is used for a clinical and radiographic evaluation of myofascial pain, disc displacement, arthralgia, arthritis, arthrosis, and Axis II, for psychologic status and pain-related disability [21, 22]. Disorders, such as orofacial myalgia or myofascial pain, disc displacement with reduction, decreased jaw movement amplitudes and side flexions to both sides, the presence of cracking or crackling of the movement, arthralgia, osteoarthrosis, were considered as TMJ-related disorders [15-20].

Clinical assessment between TMD and cervical spine abnormalities

A study by Calixtre et al. [16] observed 61 women patients who were randomly selected for interventional (IG) and control groups (CG). The IG received 10 sessions of physiotherapy over 5 weeks, twice a week within at least 48 hours apart. The following outcomes were divided into primary and secondary (pain sensitivity and functionality) outcomes. Orofacial pain intensity has significantly decreased (P<0.05) only in the IG after 5 weeks of treatment aimed at the upper cervical spine compared with the CG. A significant interplay (P<0.05) in headache impact according to HIT-6 was observed in the IG group in both time frames. No statistically significant effect was obtained for PPT of the masticatory and temporal muscles after the treatment.

Giacomo et al. [19] used the integrated DC/TMD according to the presence or not of dysfunctions of TMJ to divide patients into two subgroups. A study group consisted of patients with the following disorders, disc displacement with reduction, myalgia, and myofascial pain, subluxation, headache associated with TMD, arthralgia, and osteoarthrosis. To evaluate the relationship between TMD and cervical spine abnormalities, cephalometric assessment of C0-C1 and C1-C2 distance value, hyoid bone position, and craniocervical angle value was used. No statistically significant difference between the decrease or increase of ANB value in both groups was found.

In a study by Benlidayi et al. [18] 60 patients with TMD and with the presence or not of neck pain were evaluated according to the RDC/TMD. A significant difference between the results was found only among patients with neck pain. TMD-related masticatory efficiency (P<0.01), pain score (P<0.05), and depression score (P<0.01) were significantly higher in patients with neck pain compared to patients without neck pain. Additionally, side flexion to both sides was significantly lower (P<0.01) in patients with neck pain compared to patients without neck pain. However, functional limitation, disability points, C2-C7 angle, flexion, extension, and rotation to both sides showed no statistically significant difference between patients with or without neck pain, thus TMJ dysfunctions were not correlated with cervical abnormalities.

Hong et al. [20] classified patients into three groups: patients with no signs of TMD (control group), patients with myofascial TMD (mTMD), and patients with both myofascial TMD and cervical pain (cerTMD). The comfortable and maximum mouth opening was significantly decreased in the cerTMD group compared to the control and mTMD groups (P<0.05). TMD-related pain intensity and the number of active trigger points in masticatory and cervical muscles were considerably higher in the cerTMD group compared to the control and mTMD groups (P<0.001). The distance of the cranium-atlas (C0-C1) and atlas-axis (C1-C2) were measured to assess the head and neck posture [23]. A significant decrease in the distance of C0-C1 and C1-C2 was observed in both mTMD and cerTMD groups (P<0.05 and P<0.001 respectively), but the lowest distance was found in the cerTMD group. The reduction of the C0-C1 distance indicates posterior rotation of the cranium.

A study by Wiest et al. [17] divided 71 patients into three groups: without TMD, with low TMD, and with moderate TMD. The previously mentioned group had a significantly higher cervical lordosis and dorsal kyphosis angle (P<0.05) compared to the group without TMD and the group with low TMD. It was presented as a weak and significant correlation with the severity of TMD. No statistically significant difference between head position angle and lumbar lordosis angle was observed in cervical spine abnormalities and TMD.

In a study by Kim et al. [15] a total of 43 patients clinically diagnosed with TMD obtained a conservative treatment for more than one year. The presence of fusion abnormalities, posterior arch deficiency (PAD), or the presence of both abnormalities was considered as upper cervical spine abnormalities. An increased comfort and maximum range of mouth opening after treatment was found in all three groups, however, a significantly improved result was observed in the PAD group (P<0.05). The mouth opening limitation ranges have significantly enlarged after the treatment in the PAD and the presence of any abnormalities of TMJ groups (P<0.05). The decrease in pain on masticatory muscle palpation after the treatment in the two groups did not show a significant difference, however, a positive response was observed in the PAD group after the treatment (P=0.014). No statistically significant correlation was found in pain intensity, pain on opening, or capsule palpation between groups after the treatment.

Statistical analysis

Firstly, a systematic review and meta-analysis (qualitative and quantitative analysis) were planned. No quantitative analysis (meta-analysis) could be performed due to the high heterogeneity of the data. As a result, the systematic review only conducted a descriptive analysis of the retrieved information, without a quantitative assessment, to identify and analyze relevant data for statistical significance. Statistical data was expressed using the mean and standard deviation (M ± SD).

The goal of this systematic review was to assess the relationship between cervical spine abnormalities and TMJ dysfunctions, and to examine the functional and dysfunctional components of the craniocervical mandibular system. 8 articles were selected for this review according to predefined eligibility criteria. Due to the high methodological heterogeneity between the studies, the meta-analysis was not performed. However, the qualitative analysis revealed that decreased side flexion [18], TMD-related pain intensity, the higher number of active trigger points in masticatory and cervical muscles, reduction of C0-C1 distance [20], higher cervical lordosis and dorsal kyphosis angle [17] can significantly impact TMD related cervical abnormalities, as well as a 1-year conservative treatment can significantly reduce the TMD related symptoms [15].

Walczynska-Dragon et al. [24] study, along with clinical follow-up, indicates a high frequency of TMD coexisting with cervical spine pain. The key finding from the described study is the significant improvement in range of motion (ROM) in the cervical spine and the elimination of cervical spine pain experienced by subjects in the experimental group. Based on these study results, it becomes evident that interdisciplinary cooperation among orthopedists, laryngologists, neurologists, and dentists is necessary and essential.

The presented study did not evaluate cervical spine abnormalities and TMJ dysfunctions in relation to bruxism.  However, according to Piekartz, H.v., et al. [25], their study revealed that female subjects with bruxism have more features of myofascial TMD according to the DC/TMD consortium criteria. However, it is important to note that their study did not make direct observations regarding the relationship between bruxism and TMD. Nonetheless, the research findings demonstrated that bruxism and the severity of TMD are independent predictors for pain and cervical disability, as assessed by the Neck Disability Index (NDI) score. Interestingly, the study also found that physical measurements showed a strong relationship with pain variables, but not with variables related to ROM. These findings hold significance for clinical practice, suggesting that clinicians may encounter painful cervical clinical tests in individuals with bruxism, which may not be directly attributed to problems in the craniocervical region due to a lack of cervical impairments.

However, in our study, the treatment effects were not directly observed. It is important to note that Calixtre LB et al. [26], research has provided valuable insights. According to their findings, compared to the baseline, a significant improvement in myofascial pain and mouth opening was observed following manual therapy treatment in cases with myogenous-TMD. The short-term analysis of existing data suggests that manual therapy is an effective treatment option for TMD. Specifically, mobilization and manipulation of the upper cervical spine were found to be preferable approaches. However, it is important to acknowledge that there is limited high-quality evidence available to determine the optimal procedure or combination of manual therapy approaches and treatment duration for TMD conditions. A similar statement is made by Alan C. Lam et al. [27], who state that the current understanding of TMD lacks a standardized treatment approach, leading clinicians to employ a multimodal care strategy encompassing education, exercise, and manual therapy. In this regard, clinicians often view the cervical spine as a potential source of TMJ symptoms. Previous systematic reviews, including those conducted by Alan C. Lam et al. [27], have investigated the efficacy of various cervical manual therapy techniques for TMJ dysfunction, such as myofascial release targeting the masticatory and cervical muscles, mobilization/manipulation of the cervical and thoracic spine, and combined interventions. Furthermore, TMJ dysfunction has been associated with poor quality of sleep and exposure to stressful life events, particularly when muscular pain is involved. Clinical heterogeneity among trials may arise from factors such as psychological overlay, chronic stress, pain duration, and patient-specific beliefs and expectations regarding care. This heterogeneity may have a greater impact on subjective outcomes, which could explain the variability observed in subjective pain ratings and pressure pain thresholds (PPTs) of the masseter and temporalis muscles compared to measurements of maximal mouth opening. Nevertheless, despite small effect sizes and substantial heterogeneity, meaningful effects can still be derived from specific interventions.

Conclusion

This systematic review aimed to investigate the relationship between cervical spine abnormalities and temporomandibular joint (TMJ) dysfunctions. Decreased side flexion, increased TMD-related pain intensity, a higher number of active trigger points in masticatory and cervical muscles, and reduced C0-C1 distance were found to significantly impact TMJ-related cervical abnormalities. Additionally, conservative treatment over one year showed significant improvement in TMJ symptoms. These findings highlight the complex interplay between the cervical spine and TMJ and emphasize the need for interdisciplinary collaboration in managing patients with TMD and cervical spine pain. Further research is warranted to explore the influence of bruxism on TMD and cervical dysfunction.

Acknowledgments: We would like to acknowledge the help of Lithuanian University of Health Sciences.

Conflict of interest: None

Financial support: None

Ethics statement: This study fulfils the ethical requirements of Lithuanian University of Health Sciences.